Preparation of carboxylic acid esters

ABSTRACT

The present invention relates to a process for the preparation of higher alkyl acrylic acid esters which comprises contacting and reacting under ene reaction conditions one or more olefins with one or more alkyl acrylic acid esters in the presence of a heterogeneous catalyst prepared by reacting an alkylaluminum dichloride with silica-alumina.

FIELD OF THE INVENTION

This invention relates to a process for the preparation of carboxylic acids esters, which are known to find use, for example, in formulating medicines, ointments, cosmetics and lubricating oils, as soaps, as plasticizers, as solvents, and as chemical intermediates. More particularly, this invention relates to a reaction process for the preparation of higher alkyl acrylic acid esters, in which olefins are contacted and reacted with lower alkyl acrylic acid esters, in the presence of a heterogeneous catalyst prepared by reacting an alkyl aluminum dichloride with silica-alumina.

BACKGROUND OF THE INVENTION

Various catalysts are known to promote the "ene reaction" of olefins with alpha-, beta-unsaturated carboxylic acid esters for the production of unsaturated carboxylic acid esters. For instance, U.S. Pat. No. 3,783,136 and German Offenlengungsschrift both describe the use of AlCl₃ and AlBr₃ as catalysts for such reactions. U.S. Pat. No. 4,506,095 describes the reaction of linear alpha-olefins with alkyl acrylates catalyzed by an organometallic catalyst of the formula R_(n) --Al--X_(3--n), wherein R is an organic radical containing between about 1 and 12 carbon atoms, n is the integer 1 or 2, and X is chlorine or bromine. U.S. Pat. No. 3,641,120 describes the reaction of an ester with an olefin in the presence of a combination of a manganic carboxylic acid salt or oxide with a zirconyl carboxylic acid salt or zirconium oxide. The publication by B. R. Snider on J. Org. Chem., vol. 39, no. 2 (1972), p. 255, refers generally to Lewis acid catalysts for ene reactions, but illustrates only the use of aluminum chloride and zinc bromide. U.S. Pat. No. 3,855,255 describes the reaction of carboxylic acid esters by reacting diolefins with methacrylate esters in the presence of an organometallic complex of zero-valent nickel and an electron donor. U.S. Pat. No. 2,093,695 discloses preparation of carboxylic acid esters by reaction of acyloxy compounds with olefinic hydrocarbons catalyzed by activated charcoal, inorganic acids, the halogens and various halides of calcium, boron, cadmium, zinc, calcium and potassium. Co-pending application Ser. No. 510,309, filed Apr. 17, 1990,. U.S. Pat. No. 5,066,829 describes an ene reaction catalyzed by tantalum pentachloride. Akermark et al (J. Org. Chem., Vol. 43, No. 22 (1978), p. 4387) have reported that the eutectic mixture of AlCl3, NaCl, and KCl is a superior ene reaction catalyst. U.S. Pat. No. 3,892,788 teaches a ligand-stabilized Pt(II) dihalide complex combined with a Group IVB metal halide as a catalyst for such ene reactions. South African Patent 496,265 describes one such reaction catalyzed by various organo-metal compounds. According to German Offenlegungsschrift 2044159, a reaction between acrylic acid esters and dienes is catalyzed by an organometallic complex of zero-valent iron and a triaryl compound of an element of Group V. U.S. Pat. Nos. 3,755,386 and 4,144,257 and Belgian published application 739,208 describe similar reactions using complexes of Group VIII compounds, as well as various compounds and complexes of iron, nickel and cobalt. U.S. Pat. Nos. 4,009,203 and 3,534,087, German Offenlegungsschrift 3149979 and World Patent No. 8100846 describe related reactions of acids and olefins catalyzed by and acyloxy-stannic trihalide or a perfluorosulfonic acid resin or a crystalline metal silicate or an aluminum silicate containing a Group VIII metal compound and a polyvalent metal halide.

SUMMARY OF THE INVENTION

The present invention relates to the reaction of alkyl esters of acrylic acid with olefins promoted by a catalyst comprising a heterogeneous catalyst prepared by reacting an alkyl aluminum dichloride with silica-alumina.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention is applicable to the reaction of an olefin (formula I) with an alkyl ester of acrylic acid (formula II) for the preparation of a higher alkyl acrylic acid ester compound (formula III), as represented by the following equation: ##STR1## wherein R₁ is alkyl, R₂, R₃, R₄, R₅, R₆, R₇ and R₈ are each individually selected from the group consisting of hydrogen and alkyl moieties, and R₉ is an alkyl group.

The reactant olefins are acylic alkenes, and suitably encompass diolefins, particularly non-conjugated diolefins. In general, the olefin reactant molecule may have from 3 to about 40 carbon atoms. Preferably, the invention is applied to a liquid phase reaction involving olefins in the carbon number range of from about 6 to about 30, inclusive. In one embodiment, preference can be expressed for an olefin reactant carbon number in the range of from about 8 to about 20, inclusive, particularly a carbon number in the range of from about 14 to about 18, inclusive. The olefin molecule is suitably either branched or linear and may have either an alpha-or an internal double bond position. Olefins having a vinylidene structure have been found to be generally more reactive than olefins having a linear structure. More highly branched olefins, produces for example by oligomerization of polypropylene and butylene, are very suitable reactants.

Mixed olefin reactants are also very suitable. However, as recognized in U.S. Pat. No. 4,822,911, olefins of different molecular structure may have different reactivities under certain process conditions. Linear olefins having an internal double bond position have also been observed to react more slowly than olefins of other structures.

The lower alkyl esters of acrylic acid which are employed as reactants in this invention are suitably acrylates and alkyl-substituted acrylates represented by formula II above. Mixtures of different alkyl acrylic acid esters are suitable reactants.

The R₉ substituent of the ester reactant molecule is preferably an alkyl group having a carbon number of up to about 30, more preferably one having from 1 to about 15 carbon atoms, and most preferably one having from 1 to about 8 carbon atoms. The R₆, R₇ and R₈ substituents each independently represent either a hydrogen atom or an alkyl group, preferably a hydrogen atom or a lower, i.e., C₁ to C₄, alkyl group. If desired, the acrylate ester reactant may be suitably substituted with one or more non-hydrocarbyl substituents which do not substantially affect the intended reaction. As an example, one or more of the R₆, R₇ and R₈ substituents is suitably a halogen or a halogen substituted alkyl group.

Specific examples of alkyl acrylic acid ester reactants include methyl acrylate, ethyl acrylate, n-propyl acrylate, isopropyl acrylate, n-butyl acrylate, isobutyl acrylate, tertiary butyl acrylate, n-octyl acrylate isooctyl acrylate, 2-ethylhexyl acrylate, n-tetradecyl acrylate, n-hexadecyl acrylate and methyl alpha-chloroacrylate. In one respect, alkyl acrylic acid esters having from 4 to about 8 total carbon atoms are particularly preferred ester reactants. In another respect, preference can be expressed for ester reactants in which the R₆, R₇ and R₈ substituents are each hydrogen. Very good results have been obtained with methyl acrylate.

Also suitable as alkyl acrylic acid ester reactants in the process of the present invention are the dimers, trimers, and other oligomers of the indicated acrylic acid esters, such as, for example, trimethylolpropane triacrylate, 1,6-hexanediol diacrylate, and the like.

The relative proportions of the olefin reactant and the ester reactant are not critical to the invention. However, preference can be expressed for a molar ratio of olefin to ester reactant in the range of from about 1:30 to about 10:1. A molar ratio of olefin to ester in the range of from about 1:10 to about 10:1 is considered more preferred, and a ratio in the range of from about 1:4 to about 1:1 is considered most preferred in order to obtain good conversion to the desired product, while minimizing formation of side products.

For purposes of the process of this invention, the olefin reactant and the alkyl acrylic acid ester reactant are contacted with a catalyst prepared by reacting an alkyl aluminum dichloride, such as ethylaluminum dichloride, methylaluminum dichloride, or butylaluminum dichloride with silica-alumina, thus forming an "anchored" aluminum catalyst. As used herein, "anchored" means covalently attaching the aluminum atom to the surface by reaction of the alkylaluminum group with the surface hydroxyls. In a preferred embodiment, the catalyst is prepared by reacting ethylaluminum dichloride with silica-alumina.

In addition to the reaction and catalyst, it has been found that, while not critical, it is useful to add to the reaction mixture a small quantity of an antioxidant material such as hydroquinone, in order to inhibit free-radical initiated polymerization reactions involving the acrylate esters.

The present invention primarily relates to the use of the aluminum dichloride supported catalyst. The contact and reaction may take place in either a batch or continuous mode. In general, the process can be practiced under conditions characteristic of other ene reaction processes, although certain preferences can be expressed. Thus, the process is suitably carried out at a temperature in the range of from about 0° C. to about 300° C., preferably in the range of from about 40° C. to about 220° C., more preferably in the range of from about 60° C. to about 150° C., and most preferably in the range of from about 80° C. to about 120° C. Pressure is not a critical variable in the process of the invention, although it is desirable that the pressure be sufficient to maintain the olefin and ester reactants substantially in the liquid phase. Operation at pressures between about 0 psig and about 1000 psig are very suitable, although higher pressures, e.g., 2000 psig or greater, are also suitable. If desired, the process can be carried out using a lower carbon number olefin reactant predominantly in the vapor phase under process conditions, and a liquid phase ester reactant.

The process of the invention can be practiced in the presence of a reaction solvent such as, for example, benzene, toluene, hexane, carbon tetrachloride, ethylene chloride, ethyl acetate or other solvents recognized in the art for use in ene reactions. Solvents often enhance the reaction rate, however, solvents are not necessary for the reaction to proceed and they are inconvenient to remove from the product. Accordingly, it is generally preferred that the process be carried out in the absence of added reaction solvents.

The contact/reaction step converts the reactants, in whole or in part, to higher alkyl acrylic acid ester adducts of the olefins. The product comprises the higher alkyl acrylic acid ester adducts of the olefins represented by formula III above, as well as other higher alkyl acrylic acid ester adduct isomers. Adducts of two or more ester molecules and one olefin molecule may also be produced, particularly when the process is practiced with an excess of ester reactant. Ene reactions of olefins with acrylate esters are known to produce side products including diesters, dimers, and products from catalyst decomposition.

The reaction may be terminated by depletion of one or both of the reactants, or upon cooling of the reaction mixture, e.g., to a temperature of about 0° C. or less. Either during or following termination of the reaction, the product mixture is preferably treated for separation, e.g., by filtration, of catalyst and/or catalyst residues.

Separation of the higher alkyl acrylic acid ester products from unreacted olefin and/or ester starting materials can be accomplished by distillation or by other procedures known in the art for the processing of ene reaction products.

This invention is particularly useful when applied in connection with the process described in U.S. Pat. No. 4,822,911. For purposes of that process, a mixture of higher carbon number linear olefins and ethyl- and higher alkyl-branched vinylidene olefins is contacted with one or more alkyl esters of acrylic acid in the presence of an ene reaction catalyst, to accomplish, via selective reaction of the vinylidene olefins, both a separation of vinylidene olefins from the linear olefins and a conversion of the alkyl acrylic acid esters. The teachings of the process of U.S. Pat. No. 4,822,911 are incorporated herein by this reference.

The process of the invention is further described with reference to the following examples, which are intended to be illustrative of certain embodiments of the invention without limiting the invention's broader scope.

ILLUSTRATIVE EMBODIMENTS Preparation of Covalently Attached Aluminum Dichloride Ene Catalyst Catalyst A

Ten grams of Davison 980-25 silica alumina (60-80 mesh, calcined in air at 200° C.) and anhydrous isooctane (4 grams) were added to a round bottom flask under a nitrogen atmosphere. To this slurry, a 25% solution of ethylaluminum dichloride in hexane was titrated 1 milliliter (mL) at a time until all evolution of gas (ethane) had ceased (13 mL total amount added). The slurry was then vacuum filtered through a sintered glass funnel and the solids washed with 50 mL of isooctane, followed by four additional washes with 25 mL of isooctane. The filtrates were tested for the presence of unreacted ethylaluminum dichloride by adding several drops of isopropyl alcohol. After the third and fourth 25 mL rinses, clouding of the filtrate was no longer observed. The remaining isooctane was then removed from the solids by vacuum treatment to afford the finely divided yellow catalyst powder.

Catalyst B

Catalyst B was prepared in a manner similar to that of Catalyst A, except that 80-100 mesh silica alumina was used and calcined at 400° C. Twelve grams of silica alumina was titrated with 14 grams of 25% ethylaluminum dichloride in hexane.

EXAMPLE 1

A process according to the invention was carried out for the reaction of methyl acrylate with a C₁₁ -C₁₉ olefin reactant (propylene pentamer) having primarily a branched carbon structure. For this purpose, 20 grams of the branched olefins were contacted with 8.2 grams of methyl acrylate stabilized with hydroquinone and 5 grams of Catalyst A and placed in a 100 milliliter autoclave under a nitrogen atmosphere. After adding 100 psi of nitrogen pressure, the autoclave was heated to 100° C. over a 16 hour period. The results are presented in Table I.

EXAMPLE 2

Example 2 was carried out in a manner similar to Example 1, except that the reaction was held at 140° C. over a 16 hour period. The results of are presented in Table I.

EXAMPLE 3

A process according to the invention was carried out for the reaction of methyl acrylate with a mixture of C₁₆ olefins with carbon structures consisting of 76% linear alpha and internal olefins and 24% vinylidine olefins. 19.75 grams of this olefin mixture was contacted with 3 grams of methyl acrylate stabilized with hydroquinone and run under the following conditions intended to selectively convert the olefins with the vinylidine structure to the corresponding 1:1 ENE adducts. These reactants were added to a 100 milliliter autoclave to which 2.5 grams of Catalyst B was added. After placing the vessel under 100 psi nitrogen pressure, the autoclave was heated to 125° C. and the reaction allowed to proceed for 16 hours. The results are presented in Table I.

COMPARATIVE EXAMPLE A

Comparative Example A was carried out in a manner similar to Example 1, except that the Davison silica alumina was not pretreated with the ethylaluminum dichloride solution. This reaction was run at 80° C. and 3 grams of the silica alumina catalyst was utilized. The results are presented in Table I.

COMPARATIVE EXAMPLE B

Comparative Example B was carried out in a manner similar to Example 3, except 3 grams of the Davison silica alumina catalyst was used, and was not pre-treated with ethylaluminum dichloride. The results are presented in Table I.

                  TABLE I                                                          ______________________________________                                                %          %       %      %     %                                       Example                                                                               Unreacted  1:1     2:1    Olefin                                                                               Alkyl                                   No.    Olefins    Adduct  Adduct Dimer Chloride                                ______________________________________                                         1      76.8       21.4    1.5    0.2   ND                                      2      83.5       15.4    0.9    0.1   ND                                      3      86.9       7.6     0.6    2.4   2.5                                     Comp. A                                                                               97.6       2.3     0.1    --    ND                                      Comp. B                                                                               88.2       3.6     0.2    8.0   ND                                      ______________________________________                                     

What is claimed is:
 1. A process for the preparation of higher alkyl acrylic acid esters which comprises contacting and reacting under ene reaction conditions one or more olefins with one or more alkyl acrylic acid esters in the presence of a catalyst comprising aluminum dichloride covalently attached to silica-alumina.
 2. The process of claim 1, wherein said catalyst is prepared by reacting an alkylaluminum dichloride with silica-alumina.
 3. The process of claim 2, wherein said alkylaluminum dichloride is ethylaluminum dichloride.
 4. The process of claim 1, wherein the olefins have carbon numbers in the range of from about 6 to about 30, inclusive.
 5. The process of claim 4, wherein the reactant alkyl acrylic acid esters have carbon numbers in the range of from about 4 to about 8, inclusive.
 6. The process of claim 5, wherein the olefins have carbon numbers in the range of from about 8 to about 20, inclusive.
 7. The process of claim 6, wherein the olefins have carbon numbers in the range of from about 14 to about 18, inclusive.
 8. The process of claim 1, wherein the contact and reaction take place at a temperature in the range of from about 40° C. to about 220° C. and a pressure in the range of from about 0 psig to about 1000 psig.
 9. The process of claim 8, wherein the contact and reaction take place at a temperature in the range of from about 60° C. to about 150° C. and a pressure in the range of from about 0 psig to about 1000 psig. 